The field of the invention is x-ray apparatus for diagnostic imaging and in particular x-ray equipment having grids to reject scattered radiation.
Apparatus for creating x-ray radiographs is comprised generally of an x-ray source and an x-ray sensitive medium, such as a photographic film and screen combination, for recording an image produced by the varying transmission of x-rays directed through an imaged body.
The intensity of a radiographic image at any given point on its surface is ideally a function of the absorptive characteristics of the imaged body along a straight line from the x-ray tube to that point on the image. For this relationship to hold, x-rays that have not traveled in a straight line from the x-ray tube to the medium, i.e. those that have been scattered within the body, must be blocked to prevent their contribution to the recorded x-ray image.
Shielding the medium from scattered x-rays is typically done with a grid which is placed immediately above the medium's surface. The grid contains channels that are oriented to pass only rays proceeding in straight lines from the x-ray tube. These channels are formed by rows of parallel vanes which are constructed of an x-ray absorptive material. The vanes are separated by either a low absorptivity solid, such as plastic, or in certain instances by air gaps. Air gapped grids are used preferentially for imagining soft tissue because they attenuate the x-ray beam less and therefore provide greater contrast in the radiographic image.
The physical thickness of the grid vanes, as measured along the plane of the x-ray sensitive medium, cause some of the x-rays that would otherwise be passed by the grid, to be blocked. The blocking of these rays causes shadow "grid lines" in the image. Even fine grid lines may be distracting and larger grid lines may obscure diagnostically significant detail in the image. The problem of grid lines is most severe in grids where the vanes are spaced by air gaps as the vanes of such grids are typically thicker so that they may be self-supporting.
One method of reducing grid lines is to move the grid back and forth in the plane of the x-ray sensitive medium during the time of the x-ray exposure. The grid shadow is thus blurred by falling on different areas of the medium during the x-ray exposure. If the grid can be moved so that each area of the medium is eclipsed by a vane for an equal proportion of the exposure time, the grid lines will be effectively eliminated.
In general it is quite difficult to move the grid so that its vanes spend an equal time over each area of the medium. Reciprocating the grid at a constant speed with respect to the medium surface is one approach. Yet the goal of constant speed is upset when the grid changes direction and must be decelerated then re-accelerated in the opposite direction. With any physically realizable reciprocation, the grid vanes will spend a disproportionate amount of their time near the end of their travel as compared with the center of their travel. Accordingly, faint grid lines may appear under each vane at the vane's point of direction reversal.